The present invention relates to a block containing waste, as well as to a process for producing such a block, more particularly usable in the field of storing low and medium activity radioactive waste.
The waste from the nuclear industry can be of three different types. It can firstly consist of wet waste, such as the coprecipitation mud or sludge of liquid effluents, whose water content is approximately 20 to 40%. A second category is constituted by dry pulverulent waste e.g. formed by the incineration ash of combustible materials such as cellulose, polyvinyl chloride, rubber, neoprene, polyethylene, etc. The last category consists of so-called "technological waste", which covers the aforementioned waste types and incombustible waste, such as glass and metals.
At present, there are three main methods for coating low and medium activity radioactive waste with a view to their storage. These processes involve coating by a hydraulic binder (essentially cement), coating with bitumen and coating with polymers. Each of these methods is in principle applicable to a particular well defined type of waste and is not applicable to all the waste material types to be treated.
Treatment by cement is a method which is simple and inexpensive to carry out. However, the confinement quality is not very satisfactory when the coated product contains radioactive elements such as cesium or strontium. Industrial water leaching resistance tests have revealed that the leaching rate of such radioactive elements is high.
Coating by bitumen more particularly applies to waste materials like the sludges and concentrates of liquid effluents. This process makes it possible to obtain a stable product, but the mechanical behaviour of the coated products is not very satisfactory. Moreover, as a function of the radioisotope concentration, the product can be subject to radiolysis gas emissions, which may bring about a swelling of the coated product.
Coating in polymers consists of coating the waste in resins, e.g. thermosetting polyester resins or epoxides. The physical and mechanical properties of the product obtained, as well as the confinement are better than with coating methods in cement or bitumen. However, in certain cases, problems of compatibility between the resin matrix and the waste can be encountered, particularly in the case where it is wished to coat waste materials with a high water content. In particular, radiolysis gases are released, so that pores are produced during polymerization. Moreover, in the case of acid incinerator ash, such as e.g. those resulting from the incineration of polyvinyl chloride, there is an inhibition of polymerization due to the fact that the resin hardener may be "consumed" by the ash. Thus, in the case of epoxy resin, the hardener is basic and is attacked by the ash which is acid, which prevents the hardening of the resin.